Introduction to Arduino - Arduino Sketches: Tools and Techniques for Programming Wizardry - page 20

Your Own Arduino?
Arduino has always created open-source designs, and all the boards listed
previously have schematic i les available directly from the Arduino website,
under a Creative Commons Attribution Share-Alike license. Put simply, this
means that you are free to study the Arduino schematics to make your own or
to make modii cations either for personal use or professional use on the condi-
tion that you give credit to Arduino for the original design and release your
own design under the same license.
With the exception of the Arduino Due, all Arduino boards are based on the
Atmel AVR. These chips can be bought from electronic distributors with the
Arduino i rmware pre-installed, or if you have the proper tools, you can buy
blank chips and load the i rmware yourself.
Shields
An Arduino by itself is a capable device and already includes numerous input
and outputs, but its power only starts there. Because Arduino designs are open
source, numerous companies have developed shields, printed circuit boards
that are placed on top of the Arduino board that connect to the Arduino's pins.
There shields add functionality by using different inputs and outputs, either
digital I/O or through serial communication.
What Is a Shield?
A shield is a printed circuit board that can be placed on the top of most Arduino
boards. It connects to the Arduino's processor through male header pins. Adding
a shield to an Arduino does not necessarily expand the possibilities of an Arduino,
but most do.
For most prototyping projects, you connect wires to the Arduino's headers
and connect them to a breadboard. This is easy enough for a lot of applications,
like outputting data to two or three LEDs. For more complex applications, a
breadboard isn't practical due to the complexity of the wiring, or the size of the
components. Micro-SD card readers are extremely small and cannot be placed
onto a breadboard. Soldering wires to a micro-SD reader isn't particularly easy,
so your choices are limited. Writing data to a micro-SD card is something that
can happen a lot, so it's fortunate several companies have developed shields
with a micro-SD reader. If your application requires data logging, all you have
to do is to connect the shield to the top of the Arduino, add a few lines of code,
and you are ready to go. It is that simple.
As said previously, not all shields add functionality. Some shields exist to
help prototyping— allowing you to solder components onto the shield—without